High-temperature ovens, or reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A circular wafer or substrate, typically made of silicon, is placed on a wafer support called a susceptor. Both the wafer and susceptor are enclosed in a quartz chamber and heated to high temperatures, such as 600.degree. C. (1112.degree. F.) or higher, frequently by a plurality of radiant lamps placed around the quartz chamber. A reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes in other equipment, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits.
If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure.
Various CVD process parameters must be carefully controlled to ensure the high quality of the resulting semiconductor. One such critical parameter is the temperature of the wafer during the processing. The deposition gas reacts at particular temperatures and deposits on the wafer. If the temperature varies greatly across the surface of the wafer, uneven deposition of the reactant gas occurs.
In certain batch processors (i.e., CVD reactors which process more than one wafer at a time) wafers are placed on a relatively large-mass susceptor made of graphite or other heat-absorbing material to help the temperature of the wafers remain uniform. In this context, a "large-mass" susceptor is one which has a large thermal mass relative to the wafer. Mass is equal to the density times volume. The thermal mass is equal to mass times specific heat capacitance.
One example of a large-mass susceptor is shown in U.S. Pat. No. 4,496,609 issued to McNeilly, which discloses a CVD process wherein the wafers are placed directly on a relatively large-mass, slab-like susceptor and maintained in intimate contact to permit a transfer of heat therebetween. The graphite susceptor supposedly acts as a thermal "flywheel" which transfers heat to the wafer to maintain its temperature uniform and relatively constant. The goal is to reduce transient temperature variations around the wafer that would occur without the "flywheel" effect of the susceptor.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons including its greater precision as opposed to processing batches of wafers at the same time. Although single-wafer processing by itself provides advantages over batch processing, control of process parameters and throughput remains critical. In systems in which the wafer is supported in intimate contact with a large-mass, slab-like susceptor, the necessity of maintaining uniform susceptor temperature during heat-up and cool-down cycles limited the rate at which the temperature could be changed. For example, in order to maintain temperature uniformity across the susceptor, the power input to the edges of the susceptor had to be significantly greater than the power input to the center due to the edge effects.
Another significant problem faced when attempting to obtain high-quality CVD films is particulate contamination. One troublesome source of particulates in the CVD of metals and other conductors is the film that forms on the back side of the wafer under certain conditions. For example, if the wafer back side is unprotected or inadequately protected during deposition, a partial coating of the CVD material forms thereon. This partial coating tends to peel and flake easily for some types of materials, introducing particulates into the chamber during deposition and subsequent handling steps. One example of protecting the back side of a wafer during processing is given in U.S. Pat. No. 5,238,499 to van de Ven, et al. In this patent an inert gas is introduced through a circular groove in the peripheral region of a support platen. In U.S. Pat. No. 5,356,476 to Foster, et al., a semiconductor wafer processing apparatus is shown, including a plurality of ducts for introducing helium or hydrogen around the perimeter of a wafer to prevent flow of reactant gases downwardly into a gap between the perimeter of the wafer and a wafer support lip. The aforementioned devices, however, share the feature of rather large wafer support platens, characterized by the aforementioned detrimental high thermal mass.
Presently, there is a need for an improved wafer support system while ensuring temperature uniformity across the wafer surface.